Recently, there has existed increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, continuous studies have been performed to develop a novel electrode and battery having an improved level of capacity density and specific energy.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have a higher drive voltage and energy density than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus are spotlighted in the field of secondary batteries. However, lithium secondary batteries have a problem related to their safety, due to ignition and explosion caused by the use of an organic electrolyte. Also, lithium secondary batteries have a disadvantage in that they are obtained via a relatively complicated manufacturing process.
Evaluation of and security in safety of batteries are very important. It should be considered in the first place that users have to be protected from being damaged due to malfunctioning of batteries. To satisfy this, safety of batteries is strictly restricted in terms of ignition and combustion in batteries by safety standards. Therefore, many attempts have been made to solve safety-related problems of batteries.
In order to prevent heat emission from batteries, various methods including use of a protection circuit, use of heat occlusion by a separator, etc., have been suggested. However, use of a protection circuit causes limitation in downsizing and cost reduction of a battery pack. A mechanism of heat occlusion by a separator often acts inefficiently, when heat emission is generated rapidly. Recently, use of organic electrolyte additives has been also suggested to solve the above-mentioned problems. However, safety mechanisms based on electrolyte additives have disadvantages in that heat emission (J) may be vary depending on charging current or internal resistance of a battery, and timing is not uniform. Therefore, such safety mechanisms are always followed by degradation in the overall quality of a battery.
There have been attempts to use a polymer electrolyte in order to fundamentally solve the aforementioned problems. In general, the safety of a battery increases in the order of a liquid electrolyte<a gel type polymer electrolyte<a solid polymer electrolyte. On the other hand, the quality of a battery decreases in the same order. Because of such poor battery quality, it is known that batteries using solid polymer electrolytes have not yet been commercialized. However, more recently, Sony Corp. (Japan, see U.S. Pat. No. 6,509,123) and Sanyo Corp. (Japan, see Japanese Laid-Open Patent No. 2000-299129) have developed a gel type polymer electrolyte by a unique process and have produced batteries using the electrolyte. In brief, each gel type polymer electrolyte has the following characteristics.
In the case of the Sony's battery, PVdF-co-HFP (polyvinylidene-hexafluoropropylene copolymer) is used as a polymer, and LiPF6 dissolved in a mixed solvent of EC (ethylene carbonate) and PC (propylene carbonate) is used as an electrolyte. The polymer and the electrolyte are mixed in DMC (dimethyl carbonate) as a solvent, the resultant mixture is coated onto the surface of an electrode, and DMC is allowed to evaporate, thereby providing an electrode having a gel type polymer introduced thereto. Then, in order to provide a battery, the electrode is wound along with a polyolefin-based separator, which is used to prevent a short circuit in a battery.
Meanwhile, in the case of the Sanyo's battery, a cell is preliminarily formed by winding a cathode, an anode and a polyolefin-based separator. Next, PVdF (polyvinylidene fluoride), PMMA (polymethyl methacrylate) and PEGDMA (polyethyleneglycol dimethacrylate) and an initiator are mixed with a suitable organic carbonate mixture, and the resultant mixture is injected into the preliminarily formed cell, followed by crosslinking under suitable conditions, thereby providing a gel type polymer electrolyte. In this case, the gel type polymer electrolyte is formed in a battery in situ after the assemblage of the battery.
However, it is known that both gel type polymer electrolytes are prepared by a very complicated process and are somewhat problematic in terms of the productivity. Additionally, in most cases, the gel type polymer electrolytes improve the safety of a battery to a certain degree. However, use of the gel type polymer electrolytes is followed by degradation in the quality of a battery.